As Republicans convene less than four miles from Ground Zero, the presidential contest is crystallized by that proximity. The next four years will be the most dangerous in the nation's history, because the attacks of Sept. 11, 2001, were pinpricks compared with a clear and almost present menace. This year's pre- eminent question, beside which all others pale, is: Which candidate can best cope with the threat of nuclear terrorism?

A blood-chilling book on that is "Nuclear Terrorism: The Ultimate Preventable Catastrophe," by Graham Allison, a professor at the Kennedy School of Government at Harvard University and an adviser to John Kerry. Allison's indictment of the Iraq war -- as a dangerous distraction from and impediment to the war on nuclear terrorism he advocates -- is severable from his presentation of stark facts about the simultaneous spread of scientific knowledge and apocalyptic religious worldviews.

A dirty bomb -- conventional explosives dispersing radioactive materials that are widely used in industry and medicine -- exploded in midtown Manhattan could make much of the island uninhabitable for years. As many as one in every 100 Manhattanites might develop cancer. Perhaps even more people would die in the panic than would be killed by radiation. But even dirty bombs are relative pinpricks.

The only serious impediment to creating a nuclear weapon is acquisition of fissionable material -- highly enriched uranium (HEU) or plutonium. In 1993 U.S. officials used ordinary bolt cutters to snip off the padlock that was the only security at an abandoned Soviet-era facility containing enough HEU for 20 nuclear weapons. In 2002 enough fissile material for three weapons was recovered from a laboratory in a Belgrade suburb. Often an underpaid guard and a chain-link fence are the only security at the more than 130 nuclear reactors and other facilities using HEU in 40 countries.

Allison says that at least four times between 1992 and 1999, materials usable in weapons were stolen from Russian research institutes but recovered. How many thefts have not been reported? The U.S. Cold War arsenal included what are known as special atomic demolition munitions, which could be carried in a backpack. The Soviet arsenal often mimicked America's. Russia denies that "suitcase" nuclear weapons exist, so it denies reports that at least 80 are missing. Soviet military forces deployed 22,000 tactical nuclear warheads -- without individual identification numbers. Who thinks all have been accounted for? Russia probably has 2 million pounds of weapons-usable material -- enough for 80,000 weapons.

In December 1994 Czech police seized more than eight pounds of HEU in a parked car on a side street. A senior al Qaeda aide's proclaimed goal of killing 4 million Americans would require 1,400 Sept. 11s, or one 10-kiloton nuclear explosion -- from a softball-sized lump of fissionable material -- in each of four large American cities.

Of the 7 million seaborne cargo containers that arrive at U.S. ports each year, fewer than 5 percent are inspected. Less than 10 percent of arriving noncommercial private vessels are inspected. Given that 21,000 pounds of cocaine and marijuana are smuggled into the country each day, how hard would it be to smuggle a softball-sized lump of HEU on one of the 30,000 trucks, 6,500 rail cars or 50,000 cargo containers that arrive every day?

President Bush recently said that Democratic critics of rapid development of ballistic missile defenses are "living in the past." Perhaps. Some missile defense is feasible and, leaving aside costs, desirable. But costs cannot be left aside. Kerry, were he politically daring and intellectually nimble, might respond:

"The president is living in 1983, when Ronald Reagan proposed missile defenses to counter thousands of Soviet ICBMs. A nuclear weapon is much less likely to come to America on a rogue nation's ICBM -- which would have a return address -- than in a shipping container, truck, suitcase, backpack or other ubiquitous thing. So allocating vast amounts of scarce financial and scientific resources to missile defenses rather than other security measures is imprudent."

On the other hand, Allison argues that any hope for preventing, by diplomacy, nuclear terrorism depends on "readiness to use covert and overt military force if necessary" against two potential sources of fissile material -- Iran and North Korea. But the candidate Allison is advising has opposed virtually every use of U.S. force in his adult lifetime.

Intelligent people can differ about all that Allison says. But campaign time is becoming scarce for intelligent differing about how to prevent some American Ground Zero from becoming so poisoned by radiation that no one will be able to come within four miles of it.